Background
The authors write that "during the last 15 years, there has been an increasing focus on how climate change has and will affect the distribution and extent of ecosystems around the globe including alpine and Arctic areas (e.g., Callaghan et al., 2005)," and in this regard they report that "field studies and remote sensing have revealed a recent increase in altitude of the tree line (e.g., Kullman, 2002)," as well as "an extension and increased cover of mountain birch forest (Tommervik et al., 2009; Rundqvist et al., 2011)." More specifically, they say that Tommervik et al. have determined that "tree biomass has doubled over a 43-year period, within an area of Finnmarksvidda, and Rundqvist et al. have observed an increased density and cover of mountain birch in the treeline over the last three decades, within an area near Abisko village."

What was done
In a continuation of these types of studies, Hedenas et al. say that in 2010 they re-surveyed shrub, tree and vegetation data at 549 plots grouped into 61 clusters that were originally surveyed in 1997 in two areas close to the Abisko village, which is located approximately 200 km north of the Arctic Circle at 68°20'N, 18°50'E.

What was learned
The six Swedish scientists report that "tree basal area and biomass increased by 19% between 1997 and 2010 with the main increase occurring in established birch forest," noting that this result "concurs with the results of other studies which suggest that there has been a general increase in cover and biomass of trees and shrubs in sub-Arctic and Arctic areas," additionally citing in this regard, the studies of Sturm et al. (2001), Tape et al. (2006), Danby and Hik (2007), Forbes et al. (2010), Hallinger et al. (2010) and Van Bogaert et al. (2011).

What it means
Hedenas et al. write that in spite of the increased browsing pressure provided by an increasing reindeer population over the period of their study, as well as periodic outbursts of geometrid moths - which severely defoliated the birch trees in their study area in 2004 (Babst et al., 2010) - "there has been a net increase in biomass - and carbon drawdown - of 19%." As for the cause of this welcome phenomenon, they say "it has been suggested that increased nutrient availability associated with higher soil temperatures, and a longer growing season could underpin increased tree and shrub abundance and biomass in the Arctic (e.g., Chapin, 1983; Weih and Karlsson, 1997; Hartley et al., 1999)," as a result of "a delayed re-expansion of shrubs and trees following the 'Little Ice Age'," as suggested by Grubb (2008). And, of course, we have the ongoing and ever-increasing aerial fertilization and transpiration-reducing effects of the concomitant rise in the atmosphere's CO2 concentration, which must be playing significant roles as well, as the remarkable greening of the earth continues.

References
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Chapin III, F.S. 1983. Direct and indirect effects of temperature on Arctic plants. Polar Biology 2: 47-52.

Danby, R.K. and Hik, D.S. 2007. Variability, contingency and rapid change in recent subarctic alpine tree line dynamics. Journal of Ecology 95: 352-363.

Forbes, B.C., Fauria, M.M. and Zetterberg, P. 2010. Russian Arctic warming and 'greening' are closely tracked by tundra shrub willows. Global Change Biology 16: 152-1554.

Grubb, H. 2008. Tornetrask tree-ring width and density AD 500-2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Climate Dynamics 31: 843-857.

Hallinger, M., Manthey, M. and Wilmking, M. 2010. Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia. New Phytologist 186: 890-899.

Hartley, A.E., Neil, C., Melillo, J.M., Crabtree, R. and Bowles, F.P. 1999. Plant performance and soil nitrogen mineralization in response to simulated climate change in subarctic dwarf shrub heath. Oikos 86: 331-343.

Kullman, L. 2002. Rapid recent range-margin rise of tree and shrub species in the Swedish Scandes. Journal of Ecology 90: 68-77.

Rundqvist, S., Hedenas, H., Sandstrom, A., Emanuelsson, U., Eriksson, H., Jonasson, C. and Callaghan, T.V. 2011. Tree and shrub expansion over the past 34 years at the tree-line near Abisko, Sweden. Ambio 40: 683-692.

Sturm, M., Racine, C. and Tape, K. 2001. Climate change - increasing shrub abundance in the Arctic. Nature 411: 546-547.

Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702.

Tommervik, H., Johansen, B., Riseth, J.A., Karlsen, S.R., Solberg, B. and Hogda, K.A. 2009. Above ground biomass changes in the mountain birch forests and mountain heaths of Finnmarksvidda, northern Norway, in the period 1957-2006. Forest Ecology and Management 257: 244-257.

Van Bogaert, R., Haneca, K., Hoogesteger, J., Jonasson, C., De Dapper, M. and Callaghan, T.V. 2011. A century of tree line changes in sub-Arctic Sweden show local and regional variability and only a minor role of 20th century climate warming. Journal of Biogeography 38: 907-921.

Weih, M. and Karlsson, P.S. 1997. Growth and nitrogen utilization in seedlings of mountain birch (Betula pubescens ssp. tortuosa) as related to plant nitrogen status and temperature: A two-year study. Ecoscience 4: 365-373.